1. Field of the Invention
This invention is related to the field of Third Generation (3G) radio access network, traffic management, congestion notification and avoidance.
2. Description of the Related Art
Congestion occurs in high bit-rate radio bearers and access network systems. Congestion control is a complex issue because it is not isolated to one single level of protocol hierarchy. It is partially implemented in the routers or switches inside the network and partially in the transport protocol running on host machines.
One mechanism for congestion control deals with the allocation of resources at the destination, such as memory and processing. Generally, in accordance with flow control, the destination sets a limit on the transmission rate at which each source sending data to the destination may transmit that data. The sources and the destinations coordinate the transfer of data by an exchange of messages containing requests and acknowledgements. Before the source starts sending datagrams, such as data packets, it will send a request to the destination seeking permission to begin transmission. In response to the request, the destination send a message containing an identification of the number of packets the source may dispatch toward the destination without further authorization. This number is commonly referred to as window size. The source then proceeds to transmit the authorized number of packets toward the destination and waits for the destination successfully receives a packet, it sends a message back to the source containing an acknowledgement indicating the successful receipt of the packet and, in some cases, authorizing the source to send another packet. In this way, the number of packets on the network traveling from the source toward the destination will never be more than the authorized window size.
This mechanism, however, does not satisfactorily deal with the distribution of traffic within the network. Even with this mechanism in place, on a busy network it is likely that many sources will simultaneously send traffic over the network to more than one destination. If too much of this traffic converges on a single router in too short a time, the limited buffer capacity of the router or communication gateway will reject or destroy the packets. At this time, the network is considered to be congested.
Once the network is congested, network performance degrades significantly. The affected sources have to retransmit the lost or rejected packets. Re-transmissions, however, necessarily use network resources such as buffer storage, processing time and link bandwidth to handle old traffic, thereby leaving fewer resources for handling those portions of the messages still waiting to be transmitted for the first time. When that occurs, network delays increase drastically and network throughput drops. Since some network resources are being dedicated to handle re-transmissions at a time when the network is already experiencing a heavy load, there is a substantial risk of the congestion spreading and thereby locking up the entire network.
There are two general approaches to handle network congestion. One approach is strict resource reservation which placing limitations on the amount of traffic which will be permitted on the network at any given time. Examples include the pre-allocation of buffers at the routers to ensure that memory is available to store arriving packets until they can be forwarded. The second approach is congestion control.
In the first approach, overload is avoided through admission blocking, while the second approach trusts methods to alleviate the congestion at times when the congestion is detected. TCP/IP is a typical example of a protocol supporting the first approach, where TCP reacts by reducing its load at times when congestion is identified in the end-to-end path. TCP assumes a packet loss is an indication of an overload, which is typically the case in IP networks. Strict resource reservation of transport resources for channels may be expensive due to high peak data rates of the connections. Statistical multiplexing of bursty packet-data can result in large savings in the transport network deployment.
Resource reservation can employ the risk of transport network overload. The overload can have severe effects on the end-user performance, unless any methods to alleviate the congestion are available. This is because losses in the Transport Network Layer (TNL) will result in RLC re-transmission requests, potentially resulting in a continuous overload of the transport network. The RLC re-transmission rate would rise, the throughput would conversely plunge, but the transport network congestion would prevail. RLC “shields” the TNL losses from the end-to-end protocols, meaning that e.g. TCP will not observe the congestion related losses in the transport network.
Problems exist regarding alerting the sender of data aware of detected congestion in interfaces. Though there are means available for a receiving end of an interface to detect congestion, there are no means available to notify the sender of data or protocol data units (PDUs) about congestion.